One Power

"The One Power comes from the True Source, the driving force of creation, the force the Creator made to turn the Wheel of Time. Saidin, the male half of the True Source, and saidar, the female half, work against each other, and at the same time together to provide that force. Saidin is fouled by the touch of the Dark One, like water with a thin slick of rancid oil floating on top. The water is still pure, but it cannot be touched without touching the foulness. Only saidar is still safe to be used...The True Source cannot be used up, any more than the river can be used up by the wheel of the mill. The Source is the river; the Aes Sedai, the waterwheel."

— Moiraine Damodred to Egwene al'Vere[src]

The One Power is energy emanating from the True Source, accessible to channelers. The Power consists of manipulation of powers called elements, mainly the Five Powers. During certain points on the Wheel of Time, all five Powers can be accessed, although during others only one each is available, except to Eru Ilúvatar and usually, the Dragon.

Nature

"Those who could teach you, the last male Aes Sedai, are three thousand years dead. No Aes Sedai living can teach you to touch saidin any more than you could learn to touch saidar. A bird cannot teach a fish to fly, nor a fish teach a bird to swim."

— Moiraine Damodred to Rand al'Thor, about the differences between touching saidin and saidar[src]

Emanating from the True Source of Eru Il´vatar he One Power cannot be used up by being channeled, only woven into a purpose temporarily, and prolongs its users' lives up to (very rarely) 900. The Power's two halves, saidin and saidar, can be used by male and female channelers, respectively, except in the case of Ilúvatar, who lacks a definitive gender and is the source of the True Source, which in turn is the source of the Power. Saidin and saidar are direct opposites which complement each other and turn the Wheel of Time itself through cooperation and opposition. Due to the differences in saidin and saidar, a woman cannot teach a man to channel, or vice versa.

Channelers

Most humans are actually unable to channel the One Power at all. Some have the ability to be taught, but a small percentage are born with the "spark", an inborn ability to channel. A side effect of the spark is the relative lack of control; these channelers will soon find themselves channeling whether they want to or not.